I. Related Applications
The present application is related to a co-pending U.S. Patent Application entitled "DYNAMIC FILTERING OF PILOT ENERGY SAMPLES IN A WIRELESS COMMUNICATION SYSTEM", filed on Jun. 10, 1997 (Ser. No. 08/872,111), and assigned to the assignee of the present invention.
II. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved pilot signal detection filter for a wireless communication device which provides fast detection and validation of pilot signals while minimizing the probability of false detections.
III. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled "Personal Station--Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems." Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", U.S. Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. patent application Ser. No. 08/509,721, entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM," assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
The base station pilot signals are divided into four sets based on their pilot signal strengths as measured at the mobile station. The first set, referred to as the Active Set, comprises base stations which are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations which have been determined to be of sufficient strength to be of use to the mobile station. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined threshold T.sub.ADD. The third set is Neighbor Set which is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set which consists of all other base stations.
In an IS-95-A communication system, the mobile station sends a Pilot Strength Measurement Message when it finds a pilot of sufficient strength that is not associated with any the of the Forward Traffic Channels currently being demodulated or when the strength of a pilot that is associated with one of the Forward Traffic Channels being demodulated drops below a threshold for a predetermined period of time. The mobile station sends a Pilot Strength Measurement Message following the detection of a change in the strength of a pilot under the following three conditions:
1. The strength of a Neighbor Set or Remaining Set pilot is found above the threshold T.sub.ADD. PA1 2. The strength of a Candidate Set pilot exceeds the strength of an Active Set pilot by more that a threshold (T.sub.COMP). PA1 3. The strength of a pilot in the Active Set of Candidate Set has fallen below a threshold (T.sub.DROP) for greater than a predetermined time period.
The Pilot Strength Measurement Message identifies the base station and the measured pilot energy in decibels.
In a dynamic environment, such as when the mobile station is moving in a car, pilot signals in the active set tend to fade in strength as the mobile station moves away from them, while some pilot signals in the neighbor set tend to increase in strength as the mobile station moves toward them. Often, these changes are dramatic and sudden due to terrestrial topologies such as valleys, mountains, buildings, trees, tunnels, and other obstructions to the transmitted pilot signal. Thus, a mobile station which is moving may come out from behind an obstruction and suddenly encounter a pilot signal that is so strong that it causes interference to the mobile station, preventing it from receiving the forward link signal from the base station with which it is already communicating. This can be catastrophic to the mobile station, causing it to drop a call in progress.
However, a detection circuit that has a quick response to emerging pilots generally is more susceptible to false detections. False detections are clearly undesirable because they cause the mobile station to take actions in preparation for a handoff to a non-existent base station, thereby wasting time and resources. Thus, it is desirable to perform multiple verifications on pilot signal detections in order to prevent these false detections. However, these multiple verifications increase the time required to detect and report on emergent pilot signals. Furthermore, when there are many pilot signals to detect and verify in a serial fashion, multiple verifications of one particular pilot means that it would take longer to detect and verify other potentially strong pilot. Thus, fast response to emerging pilots and minimization of false detections are competing design considerations in the design of a pilot detection circuit for a mobile station.
In a less dynamic environment, such as when the mobile station is stationary or being carried by a pedestrian, there are times when a particular pilot signal may be approaching the threshold T.sub.ADD slowly, and vary in strength near the threshold T.sub.ADD. In this case, it is desirable to detect and promote the pilot signal from the neighbor set to the active set so that it can be used in a soft handoff. However, a detection circuit that is designed to promote stronger pilots more quickly may not be well suited to promoting weaker pilot signals, such as those that are just above T.sub.ADD. Thus, fast response to strong emerging pilots as well as weaker emerging pilots may be competing design considerations in the design of a pilot detection circuit for a mobile station.
What is needed is a pilot signal detection and verification circuit and method for its use which has a fast response to strong emerging pilot signals as well as weaker emerging pilot signals, while minimizing false detections.